SQL ALTER TABLE: Adding a Primary Key Constraint

The SQL statement ALTER TABLE customer ADD CONSTRAINT pk_id PRIMARY KEY (customer_id) will add a primary key constraint to the 'customer' table, specifically on the 'customer_id' column.

Explanation:

• ALTER TABLE: This command modifies the structure of an existing table.
• customer: This is the name of the table being modified.
• ADD CONSTRAINT pk_id: This adds a new constraint named 'pk_id' to the table.
• PRIMARY KEY (customer_id): This defines the 'customer_id' column as the primary key. A primary key ensures that each row in the table has a unique identifier, and it prevents duplicate values in the 'customer_id' column.

Therefore, the correct answer is A.